Linear motors are well known in the art. Many constructions and types have been conceived and utilized in a diverse range of applications requiring linear motion. Most constructions are frameless and consist of primary and secondary windings. Linear induction motors, for example, are commonly used in transportation, material transport and industrial machinery applications.
Linear induction motors suffer from several disadvantages which limit their use in high performance linear motion applications. The air gap required between the primary and the secondary reaction rail is very small, typically on the order of 0.002" to 0.004". This requires either that the support mechanics be of sufficient precision to maintain such clearances over the full length of travel, or the air gap be increased with the resultant loss of motor performance and efficiency. In addition, linear induction motors suffer from inherent low efficiency causing heat dissipation problems.
Much better performance can be achieved in linear motors of the type using permanent magnets. For example, Langley (U.S. Pat. No. 4,369,383) discloses a DC permanent magnet linear motor with a wound iron core. The winding can be either stationary or movable. Commutation in the Langley design is accomplished through a set of brushes which engage a commutator strip on the wound iron core.
The winding of the Langley linear motor design is located in slots between the teeth of an iron core. The permanent magnets in such a design have preferred positions of alignment corresponding to minimum reluctance, otherwise known as cogging. In higher performance applications, the cogging force can create disturbances that affect position and/or velocity accuracy. Secondly, the permanent magnets exhibit a large attractive force to the wound iron core, and therefore the moving portion must be strongly supported by the mechanical structure (i.e. bearings, etc.) to withstand the attractive force. Finally, the wound core has a large mass which often limits the available motor acceleration.
An improvement on the Langley design for many applications is disclosed by von der Heide (U.S. Pat. No. 4,151,447) where an ironless moving coil or primary is positioned between two parallel permanent magnet tracks or one magnet track and a parallel flux return member. Commutation is accomplished electronically, without brushes. The moving slider includes conventional wire-wound coils which can be made with either round or square wire. An advantage of this design is that the slider contains no ferromagnetic material and therefore has no cogging. There is also no magnetic attraction between the slider and the permanent magnet field. Furthermore, the slider, lacking iron, has a significantly lower mass. Such ironless linear motors have significant advantages over iron-core designs in high performance applications.
However, with such ironless designs the moving coil must exist in a relatively large magnetic air gap between the parallel permanent magnet field members. This large air gap causes a reduction in performance because the air gap flux density is considerably reduced. In addition, when the low mass ironless coil is the slider, the slider must be fabricated with sufficient mechanical stiffness to endure the high acceleration that the design facilitates. In the past such rigid structures have been relatively expensive to assemble.